Shake, shiver

When you shake an aerosol can it immediately feels colder, even though you are putting energy "in". Why?

• We mention two types of aerosol can here, one driven by pressurised gas, the other by a volatile, pressurised liquid in equilibrium with its vapour – Ed

• If you put one hand on a wooden surface and the other on a metal surface in a room at a stable temperature, the metal will feel colder despite both materials being at the same temperature. This is because metal is a better conductor of heat and transfers heat away from your hand faster.

When you grasp a metal aerosol can, heat is conducted from your hand to the contents touching the inside of the can. After a second or two, the can doesn't seem so cold as the temperature gradient between your hand and the contents decreases. Shake the can and fresh, cooler content comes into contact with the metal next to your hand. This increases the temperature gradient, and heat is quickly transferred from your hand and the aerosol again feels cold.

This effect does not occur if the aerosol is close to body temperature – about 37 °C – as there would then be no temperature gradient between your hand and the aerosol, so no heat flow.

The cooling effect is also felt with cans of fizzy drinks but is seldom noticed as we are trained as children not to shake them, for obvious reasons. Aerosol instructions, on the other hand, often request vigorous shaking to mix the propellant and active ingredients, so chilling is commonly felt.

The cooling from shaking is not to be confused with the coldness you get when spraying. Aerosol propellant is often liquefied gas, which readily boils at room temperature and is only liquid inside the can due to the high pressure. When the nozzle is pressed, that pressure is released and the propellant boils, taking heat from its surroundings. With continual spraying, condensation may form on the outside of the can and might even freeze.

This should not be tried indoors as some propellants are highly flammable.

David Muir, Portobello High School, Edinburgh, UK

• Inside one type of aerosol can there is the liquid that is being sprayed, such as insect repellent or hair spray, above which is a volume of high-pressure gas – the propellant. When you shake the can, you break up some of the liquid into many fine droplets, suddenly creating a much larger surface area for the liquid. This temporarily causes evaporation of some liquid into the gas.

Evaporation is an endothermic process, meaning it absorbs heat from its surroundings – in this case the liquid in the can – cooling it down. The gas space is now super-saturated, so if left alone the vapour will gradually re-condense – an exothermic, or heat-releasing, process that causes the can to return to its original temperature. But if you were able to perfectly insulate the can during this cycle, you would indeed find that it was slightly warmer at the end because of the kinetic energy put into the system by shaking the can.

Evaporative cooling is what cools you down when you sweat. If the air is 100 per cent saturated, then sweating in still air brings no relief. However, if you stand in front of a fan, even saturated air will evaporate some water molecules and cause cooling. The now super-saturated air moves away from you and will cause somewhere else to heat up when the water re-condenses.

Simon Iveson, University of Newcastle, New South Wales, Australia

• When you hold an aerosol, your hands touch the thin metal sheet surrounding the gaseous propellant. Only, it is not a gas but the vapour of a volatile liquid that is held very near its boiling point by high pressure. When you shake, liquid touches the inside of the metal and starts boiling immediately because of the warmth of your hand. The vapour will condense elsewhere in the can where your hand is not touching, releasing the heat there. The shaken can behaves somewhat like a "heat pipe", a device that combines the physics of phase transition – aka boiling and condensing – and thermal conductivity to quickly remove heat from where it is not wanted.

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